Such radiation detectors are, for example, described in the French patent FR 2 803 081 in which a sensor formed from amorphous silicon photodiodes is combined with a radiation converter.
The operation and the structure of such a radiation detector will briefly be summarized.
The photosensitive sensor is generally made from solid-state photosensitive elements that are arranged in a matrix. The photosensitive elements are made from semiconductor materials, usually monocrystalline silicon for CCD or CMOS type sensors, polycrystalline silicon or amorphous silicon. A photosensitive element comprises at least one photodiode, phototransistor or photoresistor. These elements are deposited on a substrate, generally a glass plate.
These elements are generally not directly sensitive to very short wavelength radiation, such as X-radiation or gamma radiation. For this reason, the photosensitive sensor is combined with a radiation converter that comprises a layer of a scintillating substance. This substance has the property, when it is excited by such radiation, of emitting radiation of longer wavelength, for example visible or near-visible light, to which the sensor is sensitive. The light emitted by the radiation converter illuminates the photosensitive elements of the sensor, which carry out a photoelectric conversion and deliver electrical signals that are usable by suitable circuits. The radiation converter will be called a scintillator in the rest of the description.
In order to improve the quality of a useful image, correction of the useful image is carried out based on an image called the “offset image”, or “dark image”, that is generally taken and stored at the start of an operating cycle. This offset image is the image obtained when the photosensitive device is exposed to a zero-intensity signal and corresponds to a sort of background image. The offset image varies depending on the electrical state of the components of the photosensitive elements and on the dispersion in their electrical characteristics. The useful image is that read when the photosensitive device has been exposed to a useful signal that corresponds to exposure to X-radiation. It includes the offset image. The correction consists in subtracting the offset image from the useful image.
In order to produce a useful image or an offset image, an image cycle is made, i.e. a sequence formed by an image-taking phase followed by a readout phase, then a wipe and reset phase, as explained in the patent application FR-A-2 760 585. During the image-taking phase, the photosensitive elements are exposed to a signal to be detected, whether this signal is maximum illuminance or darkness. During the readout phase, a read pulse is applied to the addressed row conductors in order to read the amount of charge that accumulated while the image was being taken. During the wipe phase, the photosensitive elements are optically wiped by means of a flash of light that is uniformly distributed across all of the photosensitive points. During the reset phase, the photosensitive elements are restored to a state in which they are receptive to taking a new image. This restoration is carried out by temporarily making a switch, switching diode or transistor element conductive by means of an electrical pulse sent over row conductors, allowing the matrix to be addressed.
Currently, the flash of light is obtained by means of a light generator that is formed from a matrix of light-emitting diodes that is placed on the back face of the detector. By convention, the front face of the detector is referred to as the face exposed to the X-radiation and the back face as the face opposite the front face. During the flash of light, the matrix of light-emitting diodes emits visible radiation that passes through the glass plate forming the substrate of the photosensitive sensor, then is reflected off faces located upstream of the photosensitive sensor 13 before reaching the photosensitive elements. A light generator may also be produced by means of lamps that are again placed on the back face of the detector.
In certain medical imaging applications in which the detector is in motion during an examination, such as tomodensitometry, for example, a thin layer of air present between the light generator and the detector may vary in thickness with the movements of the detector. The variation in this layer of air may cause the formation of a ghost image of the light generator in the images issued by the detector. This ghost image is known in the literature by the term “grid effect”.
Another drawback of light generators produced by means of light-emitting diodes or lamps resides in the thickness of such generators.
The placement of a layer of organic light-emitting diodes, referred to hereinafter as OLEDs, on the back face of the detector has also been attempted. This type of diode is made in the form of luminescent material positioned between two electrodes, at least one of which is transparent in order to allow light to be emitted outside the layer. For OLEDs, it is known practice to use a transparent electrode made of tin-doped indium oxide, referred to hereinafter as ITO for “indium tin oxide”. This type of electrode is particularly suitable for OLED layers with small surface areas. Specifically, it has been observed that ITO has a high resistivity. For an OLED layer with a large surface area, the luminosity is not homogeneous. Brightness is greater at the edges than at the center of the layer. X-radiation detectors are necessarily of large size since it is practically impossible to focus this type of radiation due to its high energy. In medical radiology, digital detectors have been produced whose dimensions may be likened to those of the silver films used in the past. Currently, there are detectors whose dimensions exceed 400 mm per side. At such dimensions, the use of an OLED layer having an ITO electrode would not allow a homogeneous flash of light to be produced.